Vacuum drying machines are known which comprise a plurality of vertically movable superimposed platforms, each of which defines a work surface with its upper face, the skin or skins to be dried being laid thereon.
Except for the lowest one, each platform is provided, on its lower face, with a cover, the perimetral edge of which can be brought into sealing contact against the underlying work surface; the cover and the underlying platform thus provide, when joined together, an evaporation chamber in which the skins being treated are enclosed.
A cover is furthermore provided above all the platforms, and is also vertically movable or fixed to the framework of the machine, and cooperates with the work surface of the uppermost platform.
To dry the skins, the work surfaces are heated by appropriate heating means and a depression is provided inside the evaporation chambers by suitable aspiration means.
In known drying machines, the platforms and the eventual movable cover are usually moved vertically by pairs of hydraulic single-action cylinders, arranged at the uprights of the framework, provided to the sides of the platforms; each cylinder is provided with its liner fixed to the base of the framework and the upper end of its stem is fixed to a support which projects from the platform, supporting it from below.
Such a system for moving the platforms is however not devoid of disadvantages, among which the main one is that, in order to achieve the required operating strokes, very long cylinders must be used, with accordingly extended liners, which entail large dimensions and are difficult to accommodate inside the machine. It is thus sometimes impossible to keep the operating positions of the various platforms at correct heights, and the overall height of the machine reaches very high values.
Such disadvantages have led to the provision of drying machines wherein the lowermost platform remains fixed, and the level on which the operator acts to remove the dried skins and lay the skins to be dried varies each time; i.e., the work level varies from a minimum height, which matches that of the fixed height of the work surface of the lowermost platform, progressively increasing as platforms are stacked, up to a maximum increase equal to the thickness of all the platforms arranged stacked in a pack.
However, in order to perform these changes in level, it is necessary to provide the drying machine with a movable stand for the operator which shifts to the same height as the work surface one is operating at, thus significantly increasing the complexity of the machine and being detrimental to the practicality of the work of the personnel.
If, instead, it is intended to provide drying machines wherein the plane on which one operates is always at the same height, it is necessary to provide, moreover, the motion of the lowermost platform, and longer strokes for the motion of the other platforms. The dimensions and the bulk of the cylinders increase accordingly, and since the fixed work height cannot be lower than the level of the upper end of the liner of the highest cylinder, either the work height is increased excessively, and so is the overall height of the machine, or the cylinders must be made to project below the base of the machine.
However, this last solution is also not devoid of disadvantages, among which the most apparent are the need to provide beforehand a number of recesses on the resting surface, suitable for accommodating the projecting portions of the cylinders, and the practical impossibility of subsequently modifying the position of the machine.
Another disadvantage which sometimes occurs in known drying machines is that the hydraulic cylinders support the platforms from below and thus, being subject to the weight of the platforms, operate in compression. For this reason, when the cylinders are completely extended, the stems project by a great length and are coupled to the liner for a limited length, so that they are often subject to bending and, generally to undesired oscillations.
Another disadvantage of known machines is the particular kind of coupling which must be provided between the upper end of the stem and the supports projecting from the platform, in order to ensure adequate resistance and reliability, even with the small size of the coupling surfaces thus obtainable.